This invention relates to solar absorbers with layers of nickel/chromium alloy and dielectric material, and is particularly applicable to solar absorbers for use on satellites and space vehicles.
The thermal equilibrium of a satellite or other vehicle travelling in space essentially depends on exchange of radiation with the surrounding environment. The satellite loses energy in the form of radiation and receives it from the sun and the earth (or the nearest planet) in a direct and/or reflected manner.
Thermal control, the role of which is to ensure maintenance of the correct operating temperatures for the various items of equipment contained in the satellite is therefore strongly, sometimes even completely, dependent on the thermo-optical properties of the coatings used (i.e. solar absorption factor .alpha..sub.s and hemispherical emission factor in the infra-red band .epsilon.). This is why it is of prime importance to have a wide range of coatings available. The coatings commonly used at present can be classified in three categories:
Cold coatings (.alpha..sub.s /.epsilon. &lt; 0.5) such as white paints (.alpha..sub.s .perspectiveto. 0.20; .epsilon..perspectiveto. 0.85),
Medium coatings (.alpha..sub.s /.epsilon..perspectiveto. 1) such as black paints (.alpha..sub.s .perspectiveto. 0.95; .epsilon..perspectiveto. 0.90) and aluminium paints (.alpha..sub.s .perspectiveto. 0.30; .epsilon..perspectiveto. 0.30),
Warm coatings (.alpha..sub.s /.epsilon.&gt; 1.5) such as polished aluminium (.alpha..sub.s .perspectiveto. 0.10; .epsilon..perspectiveto. 0.05) and polished electro-gliding (.alpha..sub.s .perspectiveto. 0.20; .epsilon..perspectiveto. 0.05).
In this last category the gilt is the warmest coating that can be provided and it has, for a long time, been desired to have available a "super warm" coating with a high .alpha..sub.s /.epsilon. ratio, that is to say having a .alpha..sub.s /.epsilon. ratio higher than 10. Such coatings are called "solar absorbers."
Various coatings can enable surfaces with a high .alpha..sub.s /.epsilon. ratio to be obtained, as follows:
"Gold-black" coating obtained by the evaporation of gold with the aid of a titanium filament heated under low helium pressure; the coatings obtained have little adhesion and are therefore not utilisable for a space application.
"Nickel-black" coating obtained from the thermal decomposition of nickel carbonyl on a hot substrates; the thermo-optical characteristics obtained are such that .alpha..sub.s /.epsilon..perspectiveto. 6, which is little better than a conventional bright electro-gilding which is much more stable.
"Electrolytic nickel-black"; the thermo-otpical characteristics obtained are very interesting (.alpha..sub.s /.epsilon..perspectiveto. 17) but the coating is difficult to reproduce and not very stable in the space environment.
Copper oxide coating obtained by the oxidation in air or in solution of an electrolytic deposit of copper; this type of coating is stable but the thermo-optical characteristics are insufficient (.alpha..sub.s /.epsilon..perspectiveto. 8-9).
Deposits on polished metallic substrates of alternated metallic and dielectric interference layers; ratios .alpha..sub.s /.epsilon. above 15 have been obtained by this method. Thus, Schmitt and Park, "Applied Optics," Vol. 4, No. 8. August 1965, pages 917-925, suggests multiple layers of magnesium fluoride, cerium oxide, aluminium and and molybdenum oxide. Certain of these deposits lead to .alpha..sub.s /.epsilon. ratios superior to 10 and are stable with respect to ultra violet rays, vacuum and elevated temperatures. The coatings described by Schmitt and Park, however, have the inconvenience of requiring the depositing of multiple layers, which makes their manufacture long and relatively expensive.
It emerges from the preceding discussion that the use of interference layers is the most satisfactory means of obtaining solar absorbers of a quality suitable for space.